Ch 5 traces the history of classifying elements — from Dobereiner and Newlands to Mendeleev and the modern periodic table. Students learn about periods, groups, and trends in atomic size, valency, and metallic character.
Dobereiner's Triads (1829): groups of 3 elements where middle element's atomic mass ≈ average of other two (e.g., Li-Na-K). Newlands' Octaves (1866): every 8th element had similar properties (worked up to calcium). Mendeleev (1869): arranged by increasing atomic mass, left gaps for undiscovered elements. Limitations: position of isotopes, irregular ordering (Co before Ni despite higher mass).
Based on atomic number (Moseley, 1913). 7 periods (horizontal) = number of shells. 18 groups (vertical) = similar valence electrons. Group 1: alkali metals (1 valence e⁻). Group 17: halogens (7 valence e⁻). Group 18: noble gases (complete outer shell). Metals on left, non-metals on right, metalloids along the staircase line.
Across a period (left → right): atomic size decreases, metallic character decreases, electronegativity increases, valency increases then decreases. Down a group (top → bottom): atomic size increases (more shells), metallic character increases, electronegativity decreases. Valency remains the same down a group.
Download: https://ncert.nic.in/textbook/pdf/jesc105.pdf | Complete book: https://ncert.nic.in/textbook/pdf/jesc1ps.zip
Noble gases (He, Ne, Ar, Kr, Xe, Rn) have completely filled outer electron shells (stable configuration). They are extremely unreactive/inert — they do not readily form compounds. They occupy Group 18 of the modern periodic table and have zero valency.
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